The present invention is directed to a method and apparatus for operational monitoring of an actuator, which apparatus includes a drive force unit that requires a characteristic time for building up the force to cause movement of the actuator, at least one position signal generator detecting a rest position of the actuator, a drive switch unit for actuating the drive force unit, and a signal connection to an analyzing means having a memory and a comparison unit.
When utilizing actuators, it is necessary, partly for safety-oriented reasons, or important for economic reasons, to monitor the functionability of the actuator in order to be able to make necessary repairs in time. For example, incorrectly tightened or worn seals, which may be in snuffing boxes, or broken drive spindles can represent sources of error. New possibilities of monitoring these sources of error occur due to the increasing spread of field bus systems.
A number of proposals for monitoring actuators are already known in conjunction with actuator devices for control and with position regulators utilized with them. These proposals are essentially based on measuring the position of the actuator relative to the drive spindle, dependent on the time and the control pressure or, respectively, flow and comparing this to, for example, initialization data.
U.S. Pat. No. 5,549,137, whose disclosure is incorporated herein by reference thereto and which was the basis for WO 95/06276, discloses a position regulator with sensor mechanism for determining the position of the actuator and for measuring the control pressure, wherein a control device is provided that generates a control signal dependent on the rated value of the actuator position, on the measured quantities and time derivations thereof This patent mentions numerous possibilities for improving the control algorithm. These possibilities are based on what is known as characteristics which are stored, particularly for the flow, and a dynamic correction of the output values or, respectively, the output of error messages as well and which characteristics are the basis of the comparison to current values. The sensor mechanism generally required for measuring the control pressure, is complicated and expensive. This is particularly true of actuators with an open/close characteristic.
German Published Application 44 19 548 discloses another method for monitoring an actuator for control having an electro-pneumatic position regulator. It is proposed therein to superimpose a variable-amplitude test signal on the control pressure and to draw conclusions about the condition of the actuator from the relationship between the test signal and the movement of the system to be controlled. The hysteresis determined according to this method by superimposition of a test signal, by contrast, is not a meaningful quantity for actuators with an open/close characteristic. Since the control pressure for actuators with the open/close characteristic only assumes two conditions, it is not possible to superimpose a variable-amplitude test signal on this device.
By comparison, proposals for monitoring actuators with the open/close characteristic are hitherto unknown. The different demands and conditions of use require new methods for monitoring that cannot be transferred from regulating control devices. An additional difficulty is based on the fact that many actuators with the open/close characteristic are utilized as safety-relevant actuators and an opening or, respectively, closing thus only occurs during an emergency. On the basis of the principle, thus, the movement of the drive spindle occurring during normal operation given regulating control devices is not suitable as a measured quantity for monitoring such actuators.